Run the following .... Reminder Text (Large Type). This is what your reminder will be. I haven't found a way yet to bypass having to run the command once before setting it on a timer. Maybe there is a way to do this, maybe not. If anyone knows, please speak up.

Now set up the timer. Run the following: Last Command (Run after Delay...) 15s

Of course, you're not limited to displaying large type with the Run after Delay... or its useful brother, Run at Time... commands, so you could use them to play an MP3, launch a document, or let your Mac speak your reminder with the "Speak Text" command (who doesn't love a little robot voice every now and then?). Personally, I've been using the large type quick reminders for a couple of weeks now, and I love setting up quick timers on-the-fly.